As the requirements of the capacity and flexibility of the optical communication system are gradually improved, the coherent optical communication technique becomes more and more important. During the coherent optical communication, the transmitted signal is simultaneously modulated in two polarization states of the light usually by means of polarization multiplexing, so as to double the transmission rate under the same bandwidth. At the receiving end, the optical coherent receiver receives signals in two polarization states simultaneously, converts them into base-band digital signals, separates the signals in two polarization states using the demultiplexing technique in the digital domain, and performs a subsequent processing of the signal in each polarization state. From the above content, it can be seen that the performance of the optical coherent receiver is greatly influenced by the fact whether the signals in two polarization states are well separated. Currently, the adaptive filter is usually used for the polarization demultiplexing (refer to Seb J. Savory, etc. “Digital filters for coherent optical receivers”, Optics Express, Vol. 16, Issue 2, pp. 804-817).
FIG. 1 illustrates a known optical coherent receiver using a digital polarization demultiplexing device. In FIG. 1, a local oscillation laser 103, polarization beam splitters 102, 104, an optical 90° mixer 109, optical-electrical converters (O/E) 110 to 113 and analog-digital converters (ADC) 114 to 117 constitute the front end of the optical coherent receiver. In FIG. 1, the reference signs 105 to 108 represent the beam-split signals. The function of the front end is to convert the received polarization multiplexed optical signal 101 into base-band electric signals 118 and 119. Due to the influence of factors such as polarization state rotation and nonlinear effect in the channel, each of the base-band electric signals 118 and 119 is a mixture of electric signals modulated in two polarization states. Thus an equalization and polarization demultiplexer 120 is required to equalize and demultiplex the base-band electric signals 118 and 119, so as to separate the signals in two polarization states to obtain signals having no crosstalk to each other, i.e., an H polarization state signal 125 and a V polarization state signal 126 as illustrated in the drawing. Next, phase restorations and data restorations of the signals are performed by an H polarization state phase restorer 121, an H polarization state data restorer 123, a V polarization state phase restorer 122 and a V polarization state data restorer 124, respectively. Generally the equalization and polarization demultiplexer 120 is implemented by an adaptive filter, whose coefficient adjustment may adopt constant modular algorithm, minimum mean square error algorithm (refer to S. J. Savory, etc., “Transmission of 42.8 Gbit/s Polarization Multiplexed NRZ-QPSK over 6400 km of Standard Fiber with no Optical Dispersion Compensation”, paper OTuA1, Proceedings of OFC 2007), etc. These algorithms use a feedback structure to adjust the coefficient of the filter according to the channel state change, so that there is no crosstalk between the H polarization state signal 125 and the V polarization state signal 126.
Existing studies show that when the polarization state change in the channel is at the KHz magnitude, the adaptive filter can track the polarization state change at this magnitude. However, due to various reasons (e.g., the nonlinear effect of the channel), the polarization state change in the channel may reach the magnitude of the signal transmission rate, i.e., the GHz magnitude. The current adaptive filter cannot track the polarization state change of such a high speed, thus crosstalk will occur between the output H polarization state signal 125 and V polarization state signal 126 due to the existence of the residual polarization scattering (refer to G. Charlet, etc., “Performance comparison of singly-polarized and polarization multiplexed at 10 Gbaud under nonlinear impairments”, OThu8, proceeding of OFC 2008). Assuming that in two polarization states, the transmitted signals are Sh and Sv, the received signals are Rh and Rv, and other signal losses have been ideally compensated. Thus, in case there is no polarization scattering between the two polarization states, Rh=Sh+nh and Rv=Sv+nv (nh and nv are noises in the H and V polarization states, respectively). In case there is any polarization scattering between the two polarization states, Rh=WhhSh+WhvSv+nh and Rv=WvvSh+WvhSv+nv (Whh, Wvv, Whv and Wvh are polarization scattering coefficients, which are all complex numbers with their amplitudes smaller than 1). By comparing the above two cases, it can be seen that the due to the polarization scattering, one polarization state signal may have a crosstalk to another polarization state signal, and since the signals modulated in two polarization states are independent from each other, the crosstalk between the two polarization state signals will certainly decrease the Signal-to-Noise Ratio (SNR) of the received signal, thereby affecting the performance of the receiver.
Thus, new method and device are required to deal with the quick polarization scattering, so as to eliminate or reduce the influence on the performance of the optical coherent receiver caused by the signal crosstalk.